Colloidal semiconductor nanocrystals are nanometer-sized fragments of the corresponding bulk crystals, which have generated fundamental interest for their promise in developing advanced optical materials. The size-dependent emission is probably the most attractive property of semiconductor nanocrystals. For example, differently sized CdSe nanocrystals can be prepared that emit from blue to red, with comparatively pure color emissions. These nanocrystal-based emitters can be used for many purposes, such as for light-emitting diodes, solid-state-lighting, solar cells through frequency down conversion, lasers, biomedical tags, and the like.
For any application of semiconductor nanocrystals as emitters, photoluminescence (PL) quantum yield (QY) is a basic and well accepted requirement. However, this previous work has not recognized that the absorption of the nanocrystals is as important as their PL QY. In applications, such as solid-state-lighting, solar cells through frequency down conversion, and some types of biomedical applications, the absorption of the nanocrystals should be as high as possible at the excitation wavelength but as low as possible at the emission wavelengths. This means that the main absorption band should reduce sharply when the wavelength is longer than the excitation wavelength, implying a separate main absorption band and emission peak. To obtain semiconductor nanocrystals with both desired emission properties and absorption properties is a challenge. This becomes even more difficult, as the focus in the field has been only on the emission properties in the past. There remains a need for nanocrystals with absorption as high as possible at the excitation wavelength and as low as possible at the emission wavelengths.
One type of useful nanocrystalline material is the core/shell nanocrystal, which features a nanocrystalline core of one type material, coated with a shell of another type material. Core/shell nanocrystals are representative of a number of different complex structured nanocrystals, such as core/shell/shell structured materials, the architectures of which are aimed at providing fine control over the nanocrystal's photophysical properties. Previously, the main goal has been to boost the emission properties, including the photoluminescence (PL) quantum yield (QY) and the photo-stability, of the nanocrystals. There remains a need for nanocrystals with optimized absorption properties of the nanocrystals through core/shell nanocrystals.
Shells of graded composition, which are multiple monolayers in thickness, are known. See, for example, Liberato Manna, Erik C. Scher, Liang-Shi Li, and A. Paul Alivasatos, “Epitaxial Growth and Photochemical Annealing of Graded CdS/ZnS Shells on Colloidal CdSe Nanorods”, J. Am. Chem. Soc., Vol. 124, No. 24, 7136-7145 (2002) (referred to herein as “Alivasatos”); Renguo Xie, Ute Kolb, Jixue Li, Thomas Basche, and Alf Mews, “Synthesis and Characterization of Highly Luminescent CdSe-Core CdS/Zn 0.5Cd0.5S/ZnS Multishell Nanocrystals”, J. Am. Chem. Soc., Vol. 127, No. 20, 7480-7488 (2005) (referred to herein as “Mews”), both incorporated herein by reference in their entirety. A graded shell composition is useful because the core and shell semiconductors generally have different lattice constants, which can cause significant lattice mismatch. Although graded shell composition are known, all graded systems are designed with the core as the central concern. The entire shell—including the graded part—is considered as a “protection layer” to boost the emission properties of the nanocrystals. See, for example, WO 2009/014707 to Kazlas, pg. 36. Such a protection layer is thought to prevent the photo-generated charges from being exposed onto the surface of the nanocrystals. The protection layer should thereby increase the photoluminescence (PL) quantum yield (QY) by offering a higher chance for the charges to recombine within the core of the nanocrystals and enhance the photostability by eliminating photochemical reactions on the surface of the nanocrystals. Papers published by the Alivisatos group and the Mews group could be considered as typical examples of such core/graded shell/shell nanocrystals.
The “Alivisatos” paper discusses growth of ZnS shell(s) onto CdSe nanorods with CdS as the graded shell between the core and outer ZnS shell to improve the photoluminescence (PL) quantum yield (QY). The photoluminescence (PL) quantum yield (QY) of the resulting CdSe/CdS/ZnS core/shell/shell nanorods is about 10-20%, which is not very high but is significantly improved in comparison to the core nanorods. The “Mews” paper, using a new growth technique (successive-layer-adsorption-and-reaction, SILAR, as described in WO 2004/066361 and U.S. Pat. Pub. No. 2007/0194279, both incorporated herein by reference in their entirety), discloses the growth of CdSe/CdS/Cd0.5Zn0.5S/ZnS core/shell/shell/shell nanocrystals to minimize the lattice mismatch between the CdSe core and the ZnS outer shell, about 12%. The photoluminescence (PL) quantum yield (QY) in “Mews” is as high as 70-85%, but the authors don't pay attention to optimizing the absorption properties. In other words, the middle CdS and Cd0.5Zn0.5S are introduced as pure “lattice matching” layers. A nanocrystal (CdSe/thick CdS/ZnS core/shell/shell) synthesized by using SILAR, as disclosed in Yongfen Chen, Javier Vela, Han Htoon, Joanna L. Casson, Donald J. Werder, David A. Bussian, Victor I. Klimov, and Jennifer A. Hollingsworth, “‘Giant’ Multishell CdSe Nanocrystal Quantum Dots with Suppressed Blinking”, J. Am. Chem. Soc., Vol. 130, No. 15, 5026-5027 (2008) (referred to herein as “Chen”), incorporated herein by reference, has quite poor emission properties, but the thick CdS shell offers excellent absorption properties although the authors did not intend the absorption properties of their nanocrystals.